1. Field of the Invention
The present invention relates generally to communications, and, more particularly, to a method and apparatus for tracking a plurality of signals received by a communications device.
2. Description of the Related Art
The telecommunications industry has undergone explosive growth over the past several years. A significant contribution to this growth has been the high demand for radio communication services, such as cordless telephone service, for example. Cordless telephones provide a greater flexibility to a user than traditional landline phones by allowing them to move freely, and not tethered to the landline telephone system.
A typical cordless telephone system includes at least one handset unit and a base unit. It is possible for a cordless telephone system to have a plurality of handset units, where each handset is capable of interfacing with a common base unit. One example of such a telephone system is the xe2x80x9cTwo Handset, One Base Systemxe2x80x9d produced by Advanced Micro Devices. The base unit is coupled to a telephone line and includes an antenna, a transmitter, and a receiver for communicating via radio frequencies with the handset unit(s). A local power line generally supplies the power for the base unit. Each handset unit of the cordless telephone system includes a speaker and a microphone, and also an antenna, a transmitter and a receiver for likewise communications with the base unit.
Under normal operating conditions, a user is able to make calls using one of the handset units that interfaces with the base unit. The user is able to roam freely about the location to make and receive calls, as long as the user stays within the generally transmitting and receiving range of the base unit. In a two-handset, one base unit telephone system, for example, the two handset units link to the single base unit over a common frequency by communicating during complementary time slots. As there are two separate handset radio systems communicating with a single base radio system, the base radio must be able to track and accommodate variations in the received signal from each handset to provide a robust link. Generally, in a two-handset, one base unit telephone system, the base station receiver is time-multiplexed between two incoming handset signals. As such, there are two different handset received analog signals that a tracking slicer circuit of the base unit is required to track, where each signal may have its own offset, amplitude, and slope.
A tracking slicer circuit compares an analog input signal against a threshold value and generates a logic level signal that is suitable for input to a digital transceiver. For robust communication, the threshold value is not fixed; instead it tracks the center of the received signal. A slicing circuit with a single tracking threshold can track the two signals from the handset units, but when switching between the two handsets it needs time to train to the relevant handset signal anomalies before the slicing circuit produces reliable digital data. Data may be corrupted or lost if the signals are different, as can been seen in FIG. 1.
FIG. 1 shows an exemplary performance of the tracking slicer circuit with a frequency offset between the received handset unit signals. Specifically, FIG. 1 illustrates an input signal provided to the tracking slicer circuit and an output signal generated by the tracking slicer circuit in response to the input signal. The input signals are the signals received from the handset units over a span of three time slots. In the first time slot, the input signal shown is received from the first handset, and in the second time slot, the input signal shown is received from the second handset. The third time slot of the input signal represents the signal received from the first handset. FIG. 1 shows an offset between received signals from two Frequency-Shift Keying (FSK) Modulated handset units, a common occurrence in multi-handset unit, one base unit telephone systems. In the presence of frequency offsets or other variations between the signals from the handset units, the tracking slicer circuit may not be able to properly track the center frequency of both handset unit signals. For this reason, as shown in FIG. 1, at least a portion of the data in the output signal is corrupted.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.
In one aspect of the present invention, a method is provided for tracking a plurality of signals in a communications device. The method includes determining a first threshold value for a first input signal of the plurality of signals and comparing the first input signal to the first threshold value to provide a first output signal. The method includes determining a second threshold value for a second input signal of the plurality of signals, and retaining at least a portion of the first threshold value while determining the second threshold value.
In another aspect of the present invention, an apparatus is provided for tracking a plurality of signals in a communications device. The apparatus includes a first tracking device, a comparator, and a second tracking device. The first tracking device is capable of determining a first threshold value for a first input signal of the plurality of signals. The comparator is capable of comparing the first input signal to the first threshold value and providing a first output signal. The second tracking device is capable of determining a second threshold value for a second input signal of the plurality of signals. The first tracking device is capable of retaining at least a portion of the first threshold value while determining the second threshold value.